a) Field of the invention
The present invention relates to an eyepiece lens system which is to be used with endoscopes such as fiber scopes, non-flexible endoscopes and so on.
b) Description of the prior art
Eyepiece lens systems for endoscopes such as fiber scopes, which are to be used for observing and photographing end surfaces of image guides, must be designed so as to satisfy the requirements (a) through (c) mentioned below:
(a) Since an image guide has a numerical aperture of at least 0.3, eyepiece lens system must be, even when a light bundle converged by an objective lens system is incident on the image guide, compatible with a numerical aperture of at least 0.2 of an image guide.
(b) An image guide generally has a diameter of 3 mm or shorter and is equipped with 50 thousand or a smaller number of picture elements arranged thereon. Since picture elements are arranged in such a small number on the end surface of the image guide and a large field angle of an eyepiece lens system will undesirably make too remarkable the mesh pattern on the end surface of the image guide fiber bundle, a wide field angle is not required for an eyepiece lens system. However, an eyepiece lens system has a small field number not exceeding 3 and must have a magnification of at least 20.times. which is higher than that of an ordinary type eyepiece lens system.
(c) An eyepiece lens system must sufficiently resolve the mesh pattern on the end surface of an image guide or have resolution of 200 lines/mm on the end surface of an image guide.
In order to satisfy the requirements described above, it is necessary to design an eyepiece lens system for endoscopes such as fiber scopes so as to have a high magnification, a large numerical aperture and favorable optical performance.
As the conventional eyepiece lens systems for endoscopes which are designed so as to satisfy the above-mentioned requirements, there are known the lens system disclosed by Japanese Patent Kokoku Publication No. Sho 60-18966.
However, this conventional eyepiece lens system is composed of four lens components consisting of six lens elements, or comprises rather large a number of lens elements. Further, this lens system adopts a lens barrel which has complicated structure and requires tedious assembly works, and is therefore expensive.